Foam cooling and acoustic damping for internal combustion engines

ABSTRACT

There is disclosed a method and apparatus for providing thermal protection and acoustic damping in an internal combustion engine by means of injecting a liquid base foam into the gas boundary zone of the combustion gases. A foaming agent, a liquid, and a gas are processed to produce a high expansion foam solution. In one embodiment the foam is generated external to the combustion chamber and is supplied to a distribution manifold located at the injector end of a rocket motor. The foam is then distributed through a plurality of orifices which direct the foam along the walls of the combustion zone and thus into the boundary layer of the combustion gases. The foam may be injected at any point along the gas boundary zone including the free jet boundary layer immediately downstream of the exhaust exit.

United States Patent Burge et a].

[ 1 Jan. 1,1974

[75] Inventors: Harland L. Burge, Tarzana; Newell C. Rodewald, ManhattanBeach,

both of Calif.

[73] Assignee: TRW lnc., Redondo Beach, Calif.

[22] Filed: Mar. 10, 1971 [21] Appl. No.: 122,858

[52] U.S. Cl 60/258, 60/39.02, 60/39.5, 50/264, 60/265, 60/310, 181/33HC, 239/265.l7 [51] Int. Cl. F02k l/26, F02k 9/02, FOln l/OO [58} Fieldof Search 60/265, 258, 264, 60/204, 39.02, 39.05, 39.5, 310, 317;

239/265.l7, 127.3; 181/33 B, 33 l-IC; 55/87,

5/1968 White 60/310 10/1970 Ayvazian ..60/204 Primary ExaminerDouglasHart Att0meyDaniel T. Anderson, William B. Leach and Donald W. Graves[57] ABSTRACT There is disclosed a method and apparatus for providingthermal protection and acoustic damping in an internal combustion engineby means of injecting a liquid base foam into the gas boundary zone ofthe combustion gases. A foaming agent, a liquid, and a gas are processedto produce a high expansion foam solution. In one embodiment the foam isgenerated external to the combustion chamber and is supplied to adistribution manifold located at the injector end of a rocket motor. Thefoam is then distributed through a plurality of orifices which directthe foam along the walls of the combustion zone and thus into theboundary layer of the combustion gases. The foam may be injected at anypoint along the gas boundary zone including the free jet boundary layerimmediately downstream of the exhaust exit.

7 Claims, 4 Drawing Figures Pmzmaum 1 mm 33821116 JET ENG'NE I I FOAM 33Fig-2 34 Harland LBurge I Newell C. Rodewold INVENTORS BY a/mi ATTORNEYFOAM COOLING AND ACOUSTIC DAMPING FOR INTERNAL COMBUSTION ENGINESBACKGROUND OF THE INVENTION This invention relates both to the coolingof internal combustion engines and to the acoustic damping of noisegenerated by the combustion gases.

The use of a liquid film to protect the walls of a rocket engine beganwith the effort on the German V-2 rocket engine. Since that time therehas been a need for improved cooling techniques brought about by the useof high energy high temperature propellants, high chamber pressures andthe need to lengthen engine life. Cooling efficiencies greater than 30percent have seldom been achieved in controlled laboratory heat transferexperiments because the liquid film is subjected to severe shear stresswhich results in wave stripping at conditions typically found in arocket motor. Wave stripping is the buildup of crests in the filmresulting in a breakdown of the cooling properties. Liquid film coolingfor rocket motors is generally recognized as an inefficient method ofprotecting the combustion chamber wall. Severe operating conditions arefound in the combustion zone of a rocket motor and therefore thermalprotection becomes of utmost importance. However, the method andapparatus disclosed herein may readily be applied to other forms ofinternal combustion engines.

The jet engine is a prime example of an internal combustion engine whichrequires some form of cooling either in the combustion zone or exhaustduct or both. The technique of cooling a rocket motor discussed hereinare applicable to jet engines and the exhaust of reciprocating pistonengines.

In addition to the thermal problem associated with the combustion thereis also the problem of noise generated by the combustion process andsometimes added to by moving machinery associatedtherewith. One focalpoint for noise abatement recently has been that of the aviationindustry. Increasing aviation requirements for higher powered engineshave resulted in an increased aero-dynamic related noise problem.Excellant progress has been made in the theoretical understanding of thesource of such noise. Attempts to alleviate the noise problem haveincluded the use of multiple nozzles, spike nozzles, slit nozzles andinternal acoustically treated ducts. An example of another approach tothe problem is that of the bypass fan engine wherein secondary air isinjected to effect mixing shear layers. Other means of alleviating jetnoise have been suggested including the injection of' water and solidparticles into the jet exhausts. The results have not indicatedsufficient noise reduction for the amount of material injected.

An example of the injection of fluids into the combustion gases of jetengines may be found in the U. S. Pat. to Wisniowski, No. 2,927,423,issued 1960. A more recent attempt of solving the noise problem is shownin the U. S. Pat. to OBrien, No. 3,442,350, issued l969 wherein amanifold assembly extends radially around and spaced from the exhaustexit of an aircraft engine. Air is mixed with soapy water to produce anannular sheet of bubbles which surrounds the exhaust gases. The bubblesheet is initially separated from the exhaust gases by the ingestion ofambient air between the manifold and exhaust duct. While O'Briendiscloses a substantial contribution to the art of noise suppression,the Applicants believe that substantial improvements in noise abatementmay be accomplished by the principles of the invention disclosed herein.

It is accordingly an object of the present invention to provide a methodand mechanism for the thermal protection and acoustic damping of noisecharacteristics associated with combustion products in internalcombustion engines which is not subject to the problems and shortcomingsof the prior art.

It is another object of the present invention to provide a method andmechanism for providing film cooling of thermal characteristics andacoustic damping of noise characteristics associated with combustionproducts wherein a liquid base foam is injected into the gas boundaryzone of the combustion products.

SUMMARY OF THE INVENTION The invention includes both method andapparatus for providing film cooling of the walls in the combustion zoneof an internal combustion engine as well as providing acoustic dampingof the noise associated with the combustion process. The Applicants havediscovered that various liquid base foams have excellant thermalinsulating and acoustic damping properties when such foams are injectedinto the gas boundary zone of the combustion gases, either in theboundary layer at or near the combustion wall or in the free jetboundary layer. The apparatus includes a foam generator of either thechemical or mechanical type for generating a foam solution from one ofseveral suitable compositions. The invention further invisions means forinjecting the foam solution directly into the gas boundary zone of thecombustion gases and may include a manifold and a plurality ofpassageways terminating substantially in the gas boundary zone. Themanifold and passageways or orifices may be located at any one ofseveral axial positions along the flow of the combustion gases, eitherwithin the combustion zone of the engines or within the exhaust duct.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram in partial cross-sectionshowing a rocket motor into which there is injected, near the injectorface, a layer of foam supplied thereto from a foam generator inaccordance with the principles of the present invention;

FIG. la is a diagram in partial cross-section showing an alternativeembodiment of means for generating and injecting a foam solution in arocket motor;

FIG. lb is another embodiment of the foam solution generation andinjection of FIGS. 1 and la;

FIG. 2 is a diagram in partial cross-section showing an alternativeembodiment in accordance with the present invention wherein foam isinjected into the exhaust ducts of an engine.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Turning now to the drawings,and with particular reference to FIG. 1, there is generally shown arocket motor 10 having a nozzle section 11, a throat section 12, acombustion section 13 and an injector assembly 14. The injector assembly14 includes a central distribution tube 15 to which fuel may be suppliedand carried into the combustion zone 16 where it is distributed radiallyoutwardly by a deflector element 17. The dis tribution tube 15 and theouter wall 18 of the injector assembly 14 form an annular passageway towhich an oxidizer may be supplied and carried into the combustion zone16 where it mixes with the fuel for combustion. An injector assembly ofthis type is generally referred to as a coaxial assembly. The rocketmotor as described is an example of one internal combustion engine withwhich the invention herein may be practiced.

FIG. 1 further shows injection means which includes a distributionmanifold 20 which is in communication with the combustion zone 16 by wayof a plurality of orifices 21. Also, shown in FIG. 1 is a foam generatorwhich includes a gas supply 22. The gas may be supplied to a liquidsupply tank 23 and a foaming agent supply tank 24 each of which isconnected to a mixing chamber 25 wherein the gas, liquid, and foamingagent are combined to form a foam solution to be supplied to thedistribution manifold 20 and then forced under pressure through theorifices 21. The foam will be carried along the walls by the combustiongases and is thereby effectively introduced into the gas boundary zoneof the combustion gases. As used herein the term gas boundary zone isused to refer both to the boundary layer of the gas as it exists betweenthe gas and a fixed boundary such as the walls of the rocket motor andalso tothe free jet boundary of the exhaust gases. The injection flowrate of the foam solution is adjusted so as to provide a continuous filmof the foam solution on the walls of the rocket motor. As thisdiscussion will develop, the advantage of the foam solution over thepure liquid as a film coolant is that the foam solution has asubstantially increased viscosity which resists shearing action by thecombustion gases and it has a highly increased surface tension over thatof pure liquid which resists gas wave interaction stripping.

Experiments conducted with a 100 lb. -thrust rocket motor effectivelyshowed that when foam was placed upon the combustion chamber wall at theinjector end, as shown in FIG. 1, the foam was found to carry throughthe subsonic, transonic, and supersonic parts of the nozzle. It was alsoobserved that the amount of fuel devoted to film cooling wassubstantially reduced. In rocket motors which introduce raw fuel alongthe combustion walls to serve as a film coolant, some to 40 percent byweight of fuel is normally required to effectively film cool the engine.The aforementioned experiments indicate that 3 to 5 percent by weight offuel was adequate as the liquid utilized in the foam generator. One ofthe phenomena associated with rocket motors is that of combustionroughness caused by combustion pressure perturbations or pressure waves.The combustion roughness is'readily evident by changes in the noiselevel associated with the operation of the rocket motor. In theforegoing experiments, the rocket motor was operated both with andwithout foam coolant. With the foam on the walls of the rocket motorthere was approximately a 40 percent reduction in the peak to peakroughness variations, thus indicating that the foam served well as anacoustic iner.

FIG. 1a is an alternative embodiment of the foregoing concept and setsforth another of a variety of ways in which the foam solution may begenerated. In FIG. 1a the foam is generated through the use of thecoaxial injector with the foaming agent being introduced directly intothe main fuel supply. In this embodiment of the invention, the fuel plusfoaming agent flows into the combustion zone where it interacts with theoxidizer flowinG through the distribution tube 15 of the injectorassembly and resulting in a flame front 26. The propellant flow ratesare adjusted such that a part of the foaming agent and fuel combinationwill pass over the primary reaction zone or flame 26 and to the wall ofthe combustion zone where the fuel and foaming agent combinationinteract with relatively cool gases. These cool gases are entrapped inthe liquid to generate a foam solution which then flows down and alongthe combustion zone wall.

FIG. lb shows yet another variation of the basic concept. In thisvariation a foaming agent and a liquid, which may be rocket fuel, isintroduced through a plurality of orifices 21. Associated with each ofthe plurality of orifices 21 is an aspirating tube or passage 27 throughwhich relatively cool gases are aspirated into the orifices 21 where thegases are entrapped in the flowing liquid and the foaming agent servesto stabilize the foam solution. As in the embodiment of FIG. 1, the foamsolution is injected along the walls of the combustion chamber 16. Allof these embodiments utilize the basic conept of generating a foamsolution which is then injected into the gas boundary zone of thecombustion gases to provide an effective means of cooling the walls ofthe rocket engine and for providing an acoustic absorber to eliminateacoustical instabilities associated with combustion. The foregoingconcepts are not limited to applications associated with rocket motorsand, in fact, are applicable to all forms of internal combustion enginesany of which may be substituted for the rocket engine shown in thefigures.

Turning now to FIG. 2 there is shown an alternative embodimentconstructed in accordance with the principles of the present inventionand shown in conjunction with ajet engine. The jet engine 30 is ofcommon design well known in the art and as assocated therewith anexhaust duct or section 31 for directing the exhaust gases associatedwith the operation of the jet engine. Associated with the exhaust duct31 is a distribution manifold 32 which is connected by a supply conduit33 to a foam generator 34. The foam generator may include the variouscomponents described in FIG. 1.

A foam solution generated by the foam generator is supplied throughconduit 33 to the manifold 32 and through a plurality of orifices 35. Afoam solution so supplied will be effectively introduced or injectedinto the gas boundary zone of the exhaust gases as they traverse theexhaust duct 31. A foam solution injected in the manner as shown in FIG.2 provides an effective means for cooling the exhaust duct as well asproviding acoustic damping of the noise associated with the operation ofthe jet engine. It may be readily observed that the jet engine 30 mayhave substituted therefore a reciprocating piston engine or any otherform of internal combustion engine.

Foam Technology The science of generating foams dates back many years.The primary application of foams in recent years has been in firefighting and the majority of literature tical purposes must beempirically determined to specific applications.

Studies of the deformation and the flow typical of foam solutions showsthat the bulk viscosity of the foam is greatly increased over that ofliquids. The effective thermal conductance of the foam is reduced fromthat of the parent liquid by virtue of entrapped low conductivity gasesthus forming an insulating blanket. The effective surface tension of thefoam is increased over that of the liquid by virtue of the entrappedbubble geometries and mechanical interlocking. The density of the foamis much lower than that of the constituent liquid. These variousproperties of foam lead to a greatly enhanced stability of the foam andresistance to free stream gas stripping. Because of these favorablequalities the quantity of material required for internal heat transferprotection can be reduced considerably below that required for pureliquid film cooling in almost direct ratio to the densities. A nominal30 to 40 percent weight of fuel is normally required in small rocketengines to effectively liquid film cool. With foam only a 3 to 5 percentby weight of fuel is necessary to provide cooling.

With regard to acoustic damping it is believed that energy dissipationis primarly twofold. First, on the surface ofthe foam the breakup of thefoam enclosed gaseous bubbles involves a certain amount of volumetricchange work and surface tension extension work. Secondly, and becausethe material is compresible and is viscously intertwined, additionalenergy is dissipated through viscous dissipation. It is known that a gasbubble in the foam tends to take on the shape of a sphere and work mustbe done to increase its surface which resists extension and has surfaceenergy. Experimental results have also shown that a resonating bubble ischaracterized by violent activity in a liquid through which intensesound wave is passing. Thus, it can be deducted that a resonating bubbleplays a dominating role in determining acoustic damping characteristics.As a general guide to the selection of foams, it may be noted that theexpansion ratio of the foam should be selected such that the averagebubble size will resonate at the predominate frequency of interest.Also, the effective viscosities should be maximized and the foam shouldbe placed in proximity of the generating noise source. The predominatetype of foaming agent that has evolved in a protein hydrolysate madefrom such materials as hoof and horn meal,- soybeans, and animal blood.Other types of foaming agents have been developed and marketed such as apolymeric film produced by agitating with air a solution of water and'alinear low molecular weight polymer which is further reacted by acatalyst in the presence of air. This foam was specifically developed tocombat fires. Another important foaming agent is a synthetic detergenttype, which when added to water, is capable of forming a high expansionfoam of approximately l,000 parts of air by volume to one part of'iquid.

The concentration of the foaming agent affects the stability and heatresistance of foam. The basic effect of the foaming agent is to reducethe surface tension of the liquid. The ratio of the volume of foam tothe volume Of liquid is another fundamental parameter and is referred toas the expansion factor. Physical properties such as thermalconductivity and foam viscosity vary widely with the expansion factor.The size of the gas bubble for a given expansion ratio governs theultimate sistance an expansion ratio of approximately 16 to I seemed tobe optimum.

Conclusions Both heat transfer and noise abatement studies have shownpositive results with the use of foam injection. Internal acousticabsorption as well as external absorption appear practical ways ofproviding acoustic damping. In the jet engine, the applicationpossibilities are numerous. With respect to compressor whine, internalsurface application of foam seems practical. The foam generator may beautomatically controlled so as to provide silencing of the jet engineonly during takeoff, landing and low altitude operation near residentialareas thereby reducing the quantity of liquid that would be needed to becarried on board theaircraft for foam generation purposes. As describedherein the Applicants have provided both a simple method and apparatusfor providing thermal protection and acoustic damping of noisecharacteristics associated with the combination product in internalcombustion engines.

What is claimed:

1. In combination with an internal combustion engine of the type havinga combination chamber therein, apparatus comprising:

a. a foam generator for producing a foam solution;

and

b. injection means in communication with said foam generator and the gasboundary zone associated with a wall of the internal combustion enginefor continuously inserting the foam solution directly into the gasboundary zone of the combustion gases during combustion.

2. The apparatus of claim 1 wherein the internal combustion engine is arocket motor of the type having an injector assembly forming a portionof the combustion chamber walls and whereinsaid injection meanscomprises:

a. a manifold substantialy encompassing the injector portion of therocket motor and'having an inlet connected to the outlet of the foamgenerator; and

b. a plurality of orifices distributed around the periphery of theinjector and each in communication with said manifold and each orificealigned so that foam injected therethrough will direct the foam alongthe walls of the rocket motor combustion chamber thereby injecting thefoam into the boundary layer of the combustion gases.

3. The apparatus of claim 1 wherein the internal combustion engine is arocket motor of the type having means for supplying rocket motor fuel tothe combustion chamber and wherein said foam generator com- Prises:

means for continuously injecting a foaming agent intO the rocket motorfuel supply at a predetermined rate such that a portion of the fuel andfoaming agent mixture willcombine with combustion gases to form a foamsolution along the walls of the combustion chamber during combustion.

4. The apparatus of claim 1 wherein the internal combustion engine is arocket motor-of the type having an injector assembly forming a portionof the combustion chamber walls and wherein said injection means ductwhereby foam may be injected into the comprises: boundary zone of theexhaust ductgases.

a plurality of orifices located proximate the combus- 6. The apparatusof claim wherein said injection tion chamber wall and the injectorassembly, and means further comprises: wherein said foam generatorcomprises: 5 a. a manifold encompassing the engine exhaust duct meansfor supplying a liquid to each orifice; and having said conduitconnected thereto; and means for supplying a foaming agent to each orib.a plurality of orifices in the wall of the engine exfice; and haust ducteach being in communication with said at least one aspirating tube incommunication bemanifold whereby foam may be injected into the tweensaid orifices and an injector face location 10 boundary zone of theexhaust gases. providing a source of relatively cool combustion 7. Amethod of providing cooling and sound absorbgases whereby a liquid and afoaming agent may ing properties for combustion gases comprising the becombined with combustion gases to form a steps of:

foam solution that flows through said orifices and a. providing aninternal combustion engine of the into the gas boundary zone. typehaving a combustion chamber therein;

5. The apparatus of claim 1 wherein the internal b. generating a liquidbase foam solution; and combustion engine is a jet engine having anexhaust c. continuously injecting the foam solution directly duct andwherein said injection means comprises: into the gas boundary zone ofthe combustion a conduit connected between and in communication chamberduring combustion.

with said foam generator and the engine exhaust

1. In combination with an internal combustion engine of the type havinga combustion chamber therein, apparatus comprising: a. a foam generatorfor producing a foam solution; and b. injection means in communicationwith said foam generator and the gas boundary zone associated with awall of the internal combustion engine for continuously inserting thefoam solution directly into the gas boundary zone of the combustiongases during combustion.
 2. The apparatus of claim 1 wherein theinternal combustion engine is a rocket motor of the type having aninjector assembly forming a portion of the combustion chamber walls andwherein said injection means comprises: a. a manifold substantiallyencompassing the injector portion of the rocket motor and having aninlet connected to the outlet of the foam generator; and b. a pluralityof orifices distributed around the periphery of the injector and each incommunication with said manifold and each orifice aligned so that foaminjected therethrough will direct the foam along the walls of the rocketmotor combustion chamber thereby injecting the foam into the boundarylayer of the combustion gases.
 3. The apparatus of claim 1 wherein theinternal combustion engine is a rocket motor of the type having meansfor supplying rocket motor fuel to the combustion chamber and whereinsaid foam generator comprises: means for continuously injecting afoaming agent intO the rocket motor fuel supply at a predetermined ratesuch that a portion of the fuel and foaming agent mixture will combinewith combustion gases to form a foam solution along the walls of thecombustion chamber during combustion.
 4. The apparatus of claim 1wherein the internal combustion engine is a rocket motor of the typehaving an injector assembly forming a portion of the combustion chamberwalls and wherein said injection means comprises: a plurality oforifices located proximate the combustion chamber wall and the injectorassembly, and wherein said foam generator comprises: means for supplyinga liquid to each orifice; means for supplying a foaming agent to eachorifice; and at least one aspirating tube in communication between saidorifices and an injector face location providing a source of relativelycool combustion gases whereby a liquid and a foaming agent may becombined with combustion gases to form a foam solution that flowsthrough said orifices and into the gas boundary zone.
 5. The apparatusof claim 1 wherein the internal combustion engine is a jet engine havingan exhaust duct and wherein said injection means comprises: a conduitconnected between and in communication with said foam generator and theengine exhaust duct whereby foam may be injected into the boundary zoneof the exhaust duct gases.
 6. The apparatus of claim 5 wherein saidinjection means further comprises: a. a manifold encompassing the engineexhaust duct and having said conduit connected thereto; and b. aplurality of orifices in the wall of the engine exhaust duct each beingin communication with said manifold whereby foam may be injected intothe boundary zone of the exhaust gases.
 7. A method of providing coolingand sound absorbing properties for combustion gases comprising the stepsof: a. providing an internal combustion engine of the type having acombustion chamber therein; b. generating a liquid base foam solution;and c. continuously injecting the foam solution directly into the gasboundary zone of the combustion chamber during combustion.